This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Carbon fixation in many ecosystems is limited by the availability of nitrogen, and N2 fixation by heterotrophic bacteria is an important source of biologically available nitrogen to terrestrial systems. An important control on nitrogen fixation in soils is the chemical speciation and bioavailability of V, Mo, and Fe because they are required metal cofactors in nitrogenase, the enzyme responsible for biological N2 fixation. Efforts to understand V cycling and bioavailability in soils is severely limited by the dearth of information on V speciation in these environments. Limited vanadium K-edge XAS data we collected recently (rapid response) at SSRL BL 4-3 demonstrated that (1) we can collect high quality XANES data even at the low V concentrations (